STP134

Introduction

For over a quarter of a century, persons interested in the field of soil mechanics have been attempting to point out and to emphasize the importance of the strength and supporting power of soil masses when they are used as an engineering material. In many cases in engineering practice, the usefulness and economic life of a structure can be measured by the supporting power of the soil upon which it rests. Unfortunately, that supporting power is difficult to determine for two important reasons. First, the nature of the soil mass itself is complex and, second, the factors which determine strength are not constant. As a consequence, the problem is one of dynamics rather than of statics. In dealing with natural soil deposits, one is essentially dealing with a three-phase soil system—a system composed of soil particles, water, and air. It is obvious that the geologic and pedologic nature of the soil particle itself will contribute to its behavior. The second and vital factor is the variable rôle played by contained water and air upon the nature and qualities of this system. This is a complicated factor. It is one which, in conjunction with the surface chemistry of the soil particle, gives the system its capacity for change and introduces the need for applying dynamic analysis methods. It follows that a reliable method for the determination of both the water content and the density of the soil mass in place is a first step toward a better understanding of the vital problem of soil strength analysis.